Bearing steels



good

Patented Bee. 31, 1946 BEARING STEELS Frederick'R. Bonte, Canton, Ohio, assignor to The Timken. Roller Bearing Company, Canton,

Ohio, a corporation of'Ohio No Drawing. Application June 9, 1944,

Serial No. 539,594

8 Claims. (Cl. .148-e31) This invention relates to graphitic steels and graphitic steel articles.

The invention is applicable particularly to the production of bearing parts, such as ball and roller bearings and cages for such bearingsand for that reason it will be described with particular reference thereto although it will be understood that it is applicable also to the production of other steel articles used in the hardened condition and in which such factors as high hardness, hardenability, wear resistance, good strength and ductility coupled with the ability to be fabricated and machined or ground satisfactorily are requisite or desirable, such, for example, as tools and dies.

Bearings hardened by quenching in oil have been produced largely from S. A. E. 52100 steel, the specification for which calls for from 0.95 to 1.1 per cent of carbon, 1.2 to. 1.5 per cent of chromium, and 0.2 to 0.5 per cent of molybdenum. Chromium is one of the so-called strategic metals, and the supply of chromium available since the advent of the present war has been inadequate to meet the. demands of both national defense and also normal demands in other fields. Consequently it would be desirable to substitute for S A. E. 52100 some other steel whose properties are not dependent upon the use of chromium, and this is so even though certain grades of the newerN. E. 52100 series use less chromium than S. A. E. 52100. However, up to the time of this invention there-has not been available, so far as I am aware, any steel in which chromium is not requisite and which is adapted tojbe substituted for S. A. E. 52100.

It is among the objects of this invention to provide steels adapted for the making of bearings and other hardened, or hardened and ground, parts of the type alluded to above which sition and easily produced and fabricated by hot working, develop desirable hardness and fine grain size together with the ability to harden deeplmfand which possess the desirable qualities free steels which are of simple composition,

possess the desirable properties of graphitic steel,

are hardenable to a hardness of 60 to 66 Rockwell C by the practice applied to S. A. E. 52100 steel, fabricate easily, and possess free machining properties.

Other objects will appear from the following description.

In my Patent No. 2,087,764, granted July 20, 1937, I have disclosed and claimed certain types of graphitic steels which have been and are now used widely, and which as defined by the claims of thepatent contain at least about 1.5 per cent of carbon. One of the steels made in accordance with that patent generally contains about 1.5 per cent of carbon, about 0.25 per cent of molybdenum, and about 0.8 per cent of silicon. That and other types of graphitic steels in accordance with the aforesaid patent have been used extensively for the making of dies, punches, and other tools which contain graphitic carbon, possess the ability to develop high hardness, are resistant to abrasion, pick up and scoring, and exhibit other desirable physical and mechanical properties, Those steels have, however, not constituted suitable substitutes for S. A. E. 52100 for the making of bearings.

I have now discovered that by substantially lowering the carbon content and increasing the contents of silicon and molybdenum of the graphitic steel specifically referred to above, and by balancing the composition so that it will graphitize upon annealing with production of not over about 0.3 per cent of graphitic carbon, there are provided graphitic steels which in addition to retaining the desirable characteristics of the do not require chromium, are of simple compo- 40 'graphitic steels and being oil hardening are suited to the production of bearings and other highly hardened parts, and may be used as substitutes for S. A. E. 52100.

More particularly, in accordance with the present invention graphitic steel'articles are made from steels containing from about 0.8 to about 1.25 per cent of carbon, about 0.2 to 0.75 per cent of molybdenum, and about 1.0 to 1.5 per cent of silicon. As just stated, these elements are balanced, in the ranges stated, to cause the steels to produce graphitic carbon when annealed, but not to exceed about 0.3 per cent thereof. To this end, in the preferred practice of the invention .the silicon is increased or decreased within the range stated as the molybdenum is increased or decreased from the lower to the upper limit named, and vice versa.

For most purposes it is preferred that the manganese content shall not exceed about 0.4 per cent; larger amounts might be used by further increasing the silicon content. It is preferred also that there be not over about 0.035 per cent each of phosphorus and sulfur.

Within such ranges I have found that for some purposes satisfactory results are had with steels containing from about 1.0 to 1.1 per cent of carbon, about 0.25 to 0.5 per cent of molybdenum, and about 1.25 per cent of silicon, and with manganese, phosphorus, and sulfur desirably as stated above. Such steels graphitize to produce not over about 0.3 per cent of graphitic carbon.

The remainder of the steels is iron together with impurities in the amounts customarily encountered in such steels, but it will be understood that other alloying elements may' be pres-- ent provided they do not detrimentally affect the properties which characterize the present invention, for which reason the remainder of the steels may besaid to be effectively iron inasmuch as such additional elements do not alter the essential character of products made in accordance with the invention.

The steels provided by the invention are preferably made in an electric furnace following standard killed steel practice, the various elements being introduced as ferro-alloys, and the composition being balanced as described above and in the manner customary in graphitic steel practice, i. e., so that graphitic carbon is not present during hot working, as described in my above-identified patent, which is an important factor. This requires casting ingots in chill molds with prompt removal of the ingots to the soaking pits to avoid formation of graphitic carbon in the ingots.

During hot working the ingots are likewise treated in a manner understood in this art, 1. e., so that the carbon will remain substantially entirely in the combined form during hot working. Generally speaking, the steels provided by this invention should be hot worked, as by rolling or forging, at temperatures higher than those used for the forging of the previously known types of graphitic steels, say at 2050 to 2200 suitably at 2050 to 2075-to 2100" F. They are converted by hot working to shapes of desired form and size for making final products. The shaped articles are then normalized and annealed to graphitize them. To this end they may be first normalized by heating above the critical range, suitably at about 1650 F., and then cooled in accordance with ordinary normalizing practice. Thereafter they areannealed by reheating them into or above the critical range, say byheating to about 1450 to l l75 F. and holding at temperature for a sufficient length of time to achieve the desired result, followed by cooling in such manner that the carbides are partially decomposed and a portion of the carbon, not over about 0.3 per cent is precipitated and dispersed throughout the structure in the form of graphitic carbon. By this heat treatment the residual carbides are spheroidized, and a feature of the invention is that both the graphitic carbon and the spheroidized carbides are not only finely divided but also they are dispersed uniformly throughout the entire structure. In fact, the graphitic carbon may be so finely dispersed that metallographic examination shows less graphitic carbon than is found by chemical analysis, which distinguishes these steels from those made commercially under my aforementioned patent.

After being graphitized the articles are machined or otherwise finished to shape and size, and they may then be subjected to a hardening treatment in which they are heated above the critical range and quenched, most suitably in oil, followed by tempering if desirable. Thereby they develop a hardness of about 60 to 66 Rockwell C which coupled with their other properties adapts them to be used as substitutes for S. A. E. 52100. The content of graphitic carbon confers lubricating properties, and the extraordinarily fine subdivision of graphitic carbon which characterizes the present steels is particularly desirable. Thus, heat treatment depends in part upon migration of carbon, and ordinarily it may happen that some of the diffusing atoms will not reach nuclei. The presence of exceedingly fine or atomic particles of graphitic carbon distributed throughout the entire structure of the steels provided by this invention is thus favorable to hardening heat treatment and renders the heat treated products more resistant to scuffing and scoring than in the case of ordinary steels and thus adapts the invention particularly to the production of ball bearings.

The steels develop high strength and ductility, as will appear from the actual data cited hereinafter, together with very fine grain size, which is desirable for bearing purposes. Moreover, they harden at least as deeply as S. A. E. 52100, which is particularly desirable for some purposes. The uniform distribution of finely spheroidized carbides throughout the structure confers free machining properties, and experience to date leads to the belief that these steels machine more readily than S. A. E. 52100. The machining and grinding properties are typically those of steels of comparable properties.

A particular advantage of the steels provided by the present invention is that they can be satisfactorily annealed in accordance with the cycle used in commercial production of bearings from S. A. E. 52100. Thus, a spheroidized and graphitized product may be produced by normalizing from 1600 F., reheating to about 1450" R, cooling through the critical range at about 10 to 20 F. per hour to 1100 F., and then discharging the parts from the furnace. This is desirable because the steels provided by this invention can thus be put into commercial production without interfering with existing practice and therefore without disturbance or slowingdown of production.

As exemplifying the invention, reference may be made to two 30 pound induction heats of steels in accordance with the invention. The analyses of these heats were as follows:

Steel C Mn Mo P Si S Par Par Per Per Per Per cent cent cent cent cent cent Heat A 1. 04 0.39 0.26 0. 015 1. 24 0.017 Heat B 99 39 52 014 1. 28 1018 The ingots were forged at 2150 to 2200 F. into 1%; inch round bars. The surface hardness of heat B as forged (388 Brinell) indicated air hardening properties. Portions of the rounds 'were normalized from 1650 F., after which the duction of S. A. E. 52100 parts. As applied to'the present steels, this annealing cycle effects graphitization. The results of applying this treatment to both normalized and unnormalized samples were as follows:

Graphitic Heat Treatment carbon Per cent Normalized-annealed..- 0.29 Unnormalized-annealed. 25 Normalized-annealed... .16 Unnormalized-annealed 13 The, average mechanical properties after annealing were as follows:

. Ultimate Heat Yield point strength Elongation Red. a1 ea P. s. i. P. 8.11 Per cent Per cent A 59, 500 100, 500 25. 5 50. 4 B 64, 000 103, 750 23. 5 47. 2

tributed uniformly. Heat B showed the same hardness and structure except that the martensite was somewhat finer. Variation in quenching temperature appeared to have n pronounced effect upon either heat.

Thehardenability of both heats was determined by standard J ominy samples quenched from 1525 F. in the J ominy quenching device. The samples were held in the furnace for one hour and the quenching interval was twenty minutes. After quenching two flat surfaces were ground to a depth of 0.015 inch 90 apart on the samples, and the Rockwell C hardnesses were measured every sixteenth of an inch. The readings were averaged and the averages were plotted together with similar data obtained with a sample of an S. A, E. 52100 production heat which contained. 1.05 per cent of carbon, 1.51 per cent of chromium, 0.33 per cent of manganese, and 0.3 per cent of silicon. This showed that the two heats in accordance with the present invention exhibited higher hardness from the surface inwardly than the commercial S. A. E. 52100 material, and that both of them hardened much more deeply, heat B being of better hardenability than heat A. The l-inch bars of heats A and B hardened throughout. This higher hardness coupled with excellent ductility is an important feature of this invention.

Similar results were obtained with a two-ton heat made in an electric furnace and having the This steel, in the form of 11-inch square ingots, forged satisfactorily at 2050 to 2075 F. into blooms of good surface quality which were hot 6 forged into. 1% inch round bars which were normalized at 1650 F. The rounds were normalized from 1650 F. and given an anneal of the type used with S. A. E. 52100. The properties obtained by these treatments were as follows:

Brinell hardness Graphitie carbon annealed, Normalized Annealed per cent The annealed hardness is greater than that of S. A. E. 52100 in the same condition. The graphitic carbon was not observable under the microscope.

The annealed bars were then turned to one-inch diameterand cut into one-half inch slices which were heat treated as described above, with the following results:

Hardness-Rockwell C Quenching I temp.

Surface Center All of' the quenched samples had a structure of fine martensite and globular carbides free from austenite, and a grain size finer than No. 10.

The hardenability of this heat was determined I likewise by the Jominy method (quenching from 1525" F.) and the results plotted in comparison with a production heat of S. A. E. 52100. As in the case of heats A and B, these tests showed this heat to have higher hardenability than S. A. E. 52100. The hardened specimens showed, by chemical analysis, about 0.09 per cent of graphitic carbon.

The properties obtained from this two-ton heat compare generally with those of the smaller heats except that the hardness was somewhat greater, due to the higher contents of molybdenum and silicon. This heat illustrates how the silicon is increased as the molybdenum content is increased. The nickel and chromium contents of this heat represent residual impurities resulting from the use of scrap, and the comparability of this heat to heats A and B shows how impurities or alloying elements may be present without detrimental effect.

This application is a continuation-impart of my copending application Serial No. 487,811, filed May 20, 1943.

According to the provisions of the patent stat-- utes, I have explained the principle and method of practicing my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. As a new article of manufacture, a hot worked article hardened by heat treatment and formed from steel containing about 0.8 to 1.25 per cent of carbon, about 0.2 to 0.75 per cent of molybdenum, about 1 to 1.5 per cent of silicon, and the remainder effectively iron, the article having dispersed throughout its structure a portion of said carbon in the formof uniformly dispersed fine carbides, and another portion as raphitic carbon but in an amount not exceeding about 0.3 per cent as the maximum amount producible by heat treatment.

2. As a new article of manufacture, a hot worked article hardened by heat treatment and formed from steel containing about 0.8 to 1.25 per cent of carbon, not over about 0.4 per cent of manganese, about 0.2 to 0.75 per cent of molybdenum, about 1 to 1.5 per cent of silicon, and the remainder effectively iron, the article as a result of heat treatment having a portion of said carbon dispersed uniformly throughout its structure in the form of fine spheroidized cementite particles, and another portion as graphitic carbon but in an amount not exceeding about 0.3 per cent as the maximum amount producible by heat treatment and being fine grained and deeply hardened.

3. As a new article of manufacture, a hot worked bearing part hardened by heat treatment and formed from steel containing about 0.8 to 1.25 per cent of carbon, not over about 0.4 per cent of manganese, about 0.2 to 0.75 per cent of molybdenum, about 1 to 1.5 per cent of silicon, not over about 0.035 per cent each of phosphorus and sulfur, and the remainder efiectively iron, the silicon content in the range stated being increased or decreased as the molybdenum is increased or decreased in the range stated, the article as a result of heat treatment having a portion of said carbon dispersed uniformly throughout its structure in the form of fine spheroidized cementite particles, and another portion as graphitic carbon but in an amount not exceeding about 0.3 per cent as the maximum amount producible by heat treatment and being fine grained and deeply hardened.

4. As a new article of manufacture, a hot worked article hardened by heat treatment and formed from steel containing about 1.0 to 1.1 per cent of carbon, not over about 0.4 per cent of manganese, about 0.25 to 0.5 per cent of molybdenum, about 1.25 per cent of silicon, not over about 0.035 per cent each of phosphorus and sulfur, and the remainder effectively iron, the article having dispersed uniformly throughout its structure a portion of said carbon in the form of fine spheroidized carbides, and another portion as graphitic carbon but in an amount not exceeding about 0.3 per cent as the maximum amount producible by heat treatment and being fine grained and deeply hardened.

5. An alloy steel containing about 0.8 to 1.25 per cent of carbon, about 0.2 to 0.75 per cent molybdenum, about 1 to 1.5 per cent of silicon, and the remainder effectively iron, said elements being balanced to produce upon heat treatment graphitic carbon in an amount not over about 0.3 per cent, and being capable upon heat treatment of developing high hardness, fine grain size, and of producing, in addition to said graphitic carbon, spheroidized carbides dispersed finely throughout the structure.

6. An alloy steel containing about 0.8 to 1.25 Per cent of carbon, about 0.2 to 0.75 per cent molybdenum, about 1 to 1.5 per cent of silicon, and the remainder effectively iron, the silicon being increased or decreased within the range stated as the molybdenum is increased or decreased Within its stated range, and said elements being balanced to produce graphitic carbon, but in an amount not over about 0.3 per cent, upon annealing, and being capable upon heat treatment of developing high hardness deeply, and of producing upon annealing, in addition to said graphitic carbon, spheroidized carbides dispersed finely throughout the structure.

7. Analloy steel according to claim 6 containing not over about 0.035 per cent each of phosphorus and sulfur.

8. An alloy steel containing about 1 to 1.1 per v cent of carbon, not over about 0.4 per cent of manganese, about 0.25 to 0.5 per cent molybdenum, about 1.25 per cent of silicon, not over about 0.035 per cent each of phosphorus and sulfur, and the remainder efiectively iron, and being capable upon heat treatment of developing high hardness deeply and fine grain size, and upon annealing of producing graphitic carbon, but in an amount not over about 0.3 per cent, and spheroidized carbides dispersed finely throughout the structure.

FREDERICK R. BONTE. 

